1. Field of the Invention
The present invention relates to an image forming apparatus, such as an electronic copying machine, provided with a driving force transmission mechanism for transmitting the driving force of a motor, used to drive an image carrier, for example, to a fixing unit or a sheet feed unit.
2. Description of the Related Art
Conventional desk-type copying machines employ a drive system in which a plurality of driven parts are driven by means of one motor, in order to enjoy reduction in power consumption, size, weight, and costs. In such a system, a single motor is used to drive an image carrier, such as a photosensitive drum, a heat roller of a fixing unit, and aligning rollers of a sheet feed unit. In this case, the driving force of the motor for driving the image carrier is transmitted to the fixing unit or the sheet feed unit by means of a driving force transmission mechanism, which includes a belt unit and a gear train.
In the image forming apparatuses of this type, residual toner particles left on the surface of the image carrier during image forming operation are removed in the following manner, by means of a cleaning unit located on the lower-course side of an image transfer position of the carrier, with respect to the rotating direction thereof. First, a cleaning blade, for use as a cleaning member, is held against the surface of the carrier in forward rotation, during the image formation, whereby the residual toner particles are scraped off. When the image forming operation is finished, the rotation of the carrier is stopped. Then, the blade is disengaged from the carrier surface, and at the same time, the carrier is rotated in reverse for a predetermined angle. As a result, the residual toner particles left between the end edge of the blade and the carrier are moved from the position of engagement between the blade and the carrier to the upper-course side of the carrier with respect to the direction of forward rotation. Thereafter, a subsequent cycle of image forming operation is performed, and residual toner particles are removed in the same manner as aforesaid. Production of black streaks is prevented by cleaning the image carrier through the operation of the carrier and the cleaning unit.
Thus, the image forming apparatuses of this type are generally constructed so that a main motor is driven in reverse to rotate the image carrier in reverse for a predetermined amount after the end of a final cycle of image forming operation. Some of these apparatuses are not provided with any means for preventing the driving force from being transmitted to the side of driven members other than the carrier, that is, the side of the fixing unit or the sheet feed unit, while the main motor is reversely rotating. In other apparatuses, the rotatory force of the main motor can be transmitted only in one direction so that the driving force cannot be transmitted to the side of the other driven members while the motor is reversely rotating.
In the former apparatuses, however, if paper sheets are jammed in the course of transportation, they return to the supply side when the image carrier rotates in reverse, causing users a great deal of frustration.
In the latter apparatuses, moreover, the driving force transmission mechanism is expensive.